On-Going Instrumentation: AO Systems

The Cerro Pachon telescope will be a superb facility for Adaptive Optics (AO) observations. A combined LGS and NGS AO system will extend and complement the AO capabilities on Mauna Kea. The combination of AO implementation at Cerro Pachon, together with the Laser Guide Star upgrade to the Phase I NGS AOS for Mauna Kea will constitute a powerful integrated AO program for the Gemini telescopes. These capabilities combined with the recommended Near IR coronagraph/imager and near IR spectroscopic capabilities will enable new scientific programs throughout the whole scope of scientific issues addressed by the workshop. Combined with our Near-IR Wavefront Sensors (WFS) for tip/tilt and fast focus correction, a LGS AO system would enable high performance AO imaging and spectroscopic observations in dark clouds - one of Gemini's key science drivers.

Our first task is to adequately characterize Cerro Pachon for AO, quantifying such parameters as C_n²(h) and wind velocity profiles. Specific site measurements, including SCIDAR measurements and a series of balloon borne atmosphere soundings, are currently underway.

On Mauna Kea, a Laser Guide Star upgrade will boost AO sky coverage very substantially at moderate Strehl ratios in the near-IR. The intent is to upgrade the initial NGS AOS (Altair) for use with a Sodium Laser beacon in order to achieve moderate Strehl ratios (around 0.5) in the near-IR over most of the sky.

Performance Guidelines for the Cerro Pachon LGS/NGS AOS:

• Wavelength Range: 1-2.5µm required; 0.5-5µm as a goal
• Operating Zenith Angle: 1-60°
• Switchable NGS <-> LGS modes
  • Emphasis on LGS but must support on-axis NGS as well
• Strehl Ratio (SR)
  • NGS - at a minimum achieve the same level as Altair
  • LGS - (SR and sky coverage at H achieved by Altair in the LGS mode
• General Spectroscopy Performance
  • LGS - TBD% sky coverage at TBD% slit throughput in a 0.1" slit at H
• Preserve location of telescope focal plane
  • Feed any instrument mounted on the Instrument Support Structure (ISS)
• Design studies may be for either altitude or telescope conjugated systems
  • Prefer capability to handle either mode
  • Issue will be sorted out when Cerro Pachon site characterization work is completed
• Optimized to support AO instrumentation
  • Coronagraphic Imager
    • Low scattering
    • Waffle mode rejection
• Apodized mask possibly integral to AO unit
  • Multi-slit/IFU Spectroscopy
    • Slit throughput
    • Boost sky coverage
    • Short wavelengths
• ~2 arcmin field of view
• Maintain at least the same throughput and emissivity specifications as Altair

Last update June 5, 1998; Ruth A. Kneale